htpR- (rpoH, sigma 32 minus) strain of E. coli harbouring the whole lux system of Vibrio fischeri is very dim. We have recently shown that GroESL proteins fully recover the expression of the lux system in this strain. This work has been undertaken to study our assumption that the GroESL proteins stabilize the LuxR protein, thus enhancing the formation of LuxR-Inducer complex. E. coli htpR- cells harbouring the luxR gene were unable to bind extracellularly added inducer, while late logarithmically growing htpR+ strain bound small quantities of the inducer. Reduction in the nutrient content of the growth medium resulted in a large increase in the capability of these cells to bind the inducer. htpR+ or htpR- E. coli strains harbouring both the luxR and the groESL genes bound large quantities of the inducer. The molecular and ecological significance of these results is discussed.